1. Field of the Invention
This invention relates to a photoemitting device and a process for forming a photocathode, which is used in forming the photocathode of, e.g., semitransparent photomultipliers (PMT).
2. Related Background Art
It is known that the photocathode emits photoelectrons outside of a photoelectron emissive material in response to incident radiation. An important consideration with the formation of the photocathode is improvement of the so-called quantum yield. The photocathode is conventionally formed by applying Sb and an alkali metal, such as, K (potassium), Cs (caesium) or others, to a glass substrate finished as a mirror. The resultant photocathode has 25 to 27% in terms of quantum yield.
The level of the quantum yield directly affects the photosensitivity of a PMT with a photocathode. Due to this fact, conventionally there have been proposed some techniques for improving the photoelectron emission efficiency. To give examples, Y. Yajima et al. have proposed in Japanese Patent Laid-Open Publication (JPLO) 37561/1974 a photocathode having an electrically conductive substance coated selectively on the coating of an optical fiber plate. Y. Watase has proposed in (JPLO) 92079/1975 a technique for forming a stepped photocathode. Gordon Peter et al. have proposed in British Patent Application No. 6701/66 a technique for forming a number of prism elements on a glass plate to improve quantum yield. J. G. Edritz et al. have proposed in U.S. Pat. No. 406183 a technique for forming rectangular concavities and convexities in a glass plate to improve photoelectron emission efficiency. In addition to these techniques, various studies have been made without being able to attain a satisfactory level.